As a structural metal, magnesium is a highly attractive alternative to other structural metals, such as aluminum and steel, in both the aerospace and automotive industries. In particular, magnesium is light weight, has the highest strength-to-weight ratio of any structural metal, is machinable, and is dimensionally stable. However, an impediment to the widespread use of magnesium is that it is relatively expensive to produce.
It is known in the art to electrolytically produce magnesium metal through the use of a molten salt bath containing magnesium chloride (MgCl.sub.2) and various other salts, such as calcium chloride (CaCl.sub.2) and sodium chloride (NaCl). Conventionally, the magnesium chloride is electrolytically decomposed to produce magnesium metal (Mg) on a steel cathode and chlorine gas (Cl.sub.2) on a graphite anode at temperatures between about 700.degree. C. and about 740.degree. C.
While the particular processes employed to produce magnesium metal vary within the relevant industry, a primary difference in the processes is the purity of the magnesium chloride used and the techniques employed for preparing the magnesium chloride. As an example, partially dehydrated magnesium chloride is used in one well known process, while anhydrous magnesium chloride is used in another. Natural resources of magnesium include seawater, salt lakes and underground brine and salt beds, as well as minerals such as magnesite (MgCO.sub.3), dolomite (CaMg(CO.sub.3).sub.2), carnallite (KCl.MgCl.sub.2.6HOH or KMgCl.sub.3.6HOH) and brucite (Mg(OH).sub.2), each of which requires different processing procedures to procure substantially pure magnesium chloride.
While the purity or source of the magnesium chloride may differ, it is the preparation of the magnesium chloride feed stock which forms the economic burden to the electrolytic process. The U.S. Department of Energy, Final Report EX-76-A-01-2295 (1981), entitled "An Assessment of Magnesium Primary Production Technology", M.C. Flemings et al., reported that about fifty percent of the total cost and energy consumption for the production of magnesium is consumed in the preparation of the magnesium chloride. The high cost of using "pure" magnesium chloride as a feed stock is substantially a result of the numerous processing steps necessary for its preparation, which include, depending on the method adopted, precipitation, filtration and calcination, pelletization, chlorination, and alternatively neutralization and dehydration processes. Accordingly, a significant economic impediment to the production and use of magnesium would be removed if the process did not rely on the use of magnesium chloride as the feed stock.
In the preparation of magnesium chloride, magnesium oxide (MgO) is a common impurity that is highly undesirable in the electrolyte bath. Because magnesium oxide is only slightly soluble in the conventional electrolytes known in the art, it remains suspended throughout the process, causing a sludge to form within the electrolyte. As a result, some elemental magnesium is lost during the process as sludge, which must be periodically removed from the electrolytic cell.
Even if magnesium oxide is substantially removed as an impurity by the manner in which magnesium chloride is conventionally refined, the undesirable sludge still forms because magnesium chloride reacts with the water and/or moisture present in the other constituents of the electrolyte bath to form magnesium oxide. Accordingly, the formation of sludge is likely to occur regardless of the purity of the magnesium chloride used as the feed stock to the electrolytic process.
From the above, it is readily apparent that presently known electrolytic methods for producing magnesium metal entail an involved process for procuring and purifying magnesium chloride from natural sources. Furthermore, a primary impurity, magnesium oxide, is highly undesirable within known electrolytic processes in that it results in the loss of some elemental magnesium through the formation of a sludge. Finally, this sludge has a tendency to form even when highly pure magnesium chloride is used as the feed stock to the process because of the ability for magnesium oxide to form by the reaction of magnesium chloride with water and/or moisture present in the other constituents of the electrolyte bath.
Thus, it would be desirable to provide a more economical method for producing magnesium metal which does not require pure magnesium chloride as the feed stock for the electrolysis process, and prevents the formation of suspended magnesium oxide in the electrolyte during the electrolysis process.